Method of making electron emitters



P. ,D. WILLIAMS 2,693,431 METHOD OF MAKING ELECTRON EMITTERS Nov. 2, 1954 Filed Jan. 27, 1948 CATHODE FABR/CATED I 01- awn/0244750 METAL SUCH A5 TUNESTEN CATHODE CLEANED A5 BY FIR/N6 IN VACUUM CATHODE COATED WITH THOR/UM COMPOUNJ 506/1 14$ THOR/UM HYDE/0E CAT/100E HEATED IN A LOW PRESSURE ATMOSPHERE CONT/4 lN/NG OXYGEN CATHODE HEATEU IN PRESENCE OF CARBON 'INVEN TOR. Paul B Will/a 5 ATTOE/VE Y United States Patent Ofiice 2,693,431 Patented Nov. 2, 1954 METHQD OF MAKING ELECTRON EMITTERS Paul D. Williams, Palo Alto, Calif., assignor to Eitel- McCullough, Inc., San Bruno, Calif., a corporation of California Application January 27, 1948, Serial No. 4,648

2 Claims. (Cl. 148-13.1)

My invention relates to an improved method of making a thoriated type emitter for electron tubes.

A thoriated type of emitter comprising a metal such as tungsten impregnated with thoria (thorium oxide) is well known in the art, thoriated tungsten wire having been used for many years in the making of filaments for electron tubes. Likewise, the advantages of a thoriated tungsten filament from the standpoint of long life and resistance to adverse influences such as contamination and bombardment are well known. Heretofore the use of thoriated emitters has been largely limited to filamentary structures.

Tungsten containing around 1% thoria is produced by the wire manufacturer and supplied as such to the electron tube making industry. Because of the metallurgical problems involved in the making and handling of thioriated tungsten it is commercially available only in the wire form and its use has therefore been confined to the making of filaments, a variety of filamentary structures including helical and parallel bar filaments having been developed over the years to make use of thoriated tungsten wire.

The principal object of my invention is to provlde a method of making a thoriated type emitter which materially extends its field of use and enables the tube manufacturer to fabricate such emitters in sheet form for unipotential surface cathodes, such as indirectly heated cathodes and the like.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of species of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

The single figure is a flow diagram illustrating the preferred steps in my process.

In terms of broad inclusion my method of making a thoriated type emitter comprises forming the emitter body of an unthoriated refractory metal such as substantially pure tungsten, then thoriating the body, and then carburizing it. Since the emitter body is formed or shaped of unthoriated metal it may be readily fabricated and welded into any desired shape such as 2. cylinder or the like. The thoriating procedure preferably comprises coating the metal body with a thorium compound such as thorium hydride, and then heating the body and exposing it to oxygen, the latter being preferably accomplished by heating the body in a low pressure atmosphere containing oxygen. During the last mentioned step the thorium hydride decomposes under heat and the metallic thorium penetrates the metal body and is converted to thorium oxide. The subsequent carburizing step involves converting at least a portion of the body metal to a metallic carbide such as tungsten carbide. This may be accomplished by heating the body in an atmosphere containing carbon, or, in the case of some structures, is preferably accomplished by coating the body with carbon and then heating it. The emitter body so processed is ready for mounting in a tube envelope. While the method embodying my invention is particularly well adapted for making thoriated unipotential surface type cathodes, it may also be used in making filamentary types.

In greater detail, and referring to the drawing, my method of making an electron emitter comprises first forming the emitter body of a suitable unthoriated refractory metal such as tungsten or tantalum, tungsten being preferred. An ordinary commercial grade of substantially pure tungsten is satisfactory. If a cylindrical emitter body is desired, such a structure is readily made by rolling tungsten sheet into a cylinder and butt welding it at the seam. I have made cylindrical cathodes of this kind up to several inches in diameter from 20 mil tungsten sheet. Since the tungsten sheet is not thoriated at this stage it may be procured, shaped, welded and otherwise handled in the ordinary manner. A wide variety of cathode shapes are possible, sleeves, cups, disks, and those having planar as well as curved emitter surfaces. Filaments may also be made, using unthoriated tungsten wire.

After the emitter body has been formed it is preferably cleaned to remove any surface contamination such as grease or the like. Cleaning by heating the parts in vacuum is satisfactory. In the case of a tungsten body I prefer to heat it to about 2000 C. brightness temperature in a vacuum furnace.

The emitter body or cathode is then coated with a thorium compound adapted to decompose to produce thorium metal when heated. The principal requirements here are that the compound disassociate rather than volatilize under heating (so that metallic thorium is provided), and that the disassociated radicle of the compound be volatile (so as to leave the surface being treated). Thorium hydride works very well and is preferred, although other thorium compounds are suggested such as thorium iodide, thorium sulphide and thorium nitride. The thickness of coating does not appear critical, although complete coverage of the underlying metal is desirable. Application of the coating by spraying is convenient, in which case the thorium hydride is preferably suspended in an amyl acetate-gun cotton lacquer of the proper consistency for spraying.

The coated body is then heated and exposed to oxygen to decompose the thorium hydride, effect penetration of the thorium, and conversion of the thorium to thorium oxide. This may all be done in one operation by heating the body in a low pressure atmosphere containing oxygen, as in a vacuum furnace having a small amount of oxygen present. A bell jar type vacuum furnace pumped down to a reasonably good vacuum is satisfactory because the residual gases in the system will include enough oxygen for purposes of oxidizing the thorium. In the case of a tungsten body coated with thorium hydride I prefer to heat the body to about 2100 C. brightness temperature. The body is preferably brought up to this temperature as rapidly as possible and then held at that temperature for about two minutes.

During the firing step the thorium hydride decomposes, producing thorium metal and releasing hydrogen. The thorium penetrates the tungsten body by migration along the intercrystalline faces, and, since this penetration is probably most effective when the thorium is in the metal state, it is desirable to cause speedy disassociation of the thorium hydride so that the rapid evolution of hydrogen will sweep away any oxygen in the atmosphere adjacent the surface. Because of the high afiinity thorium has for oxygen, the thorium will oxidize as quickly as any oxygen gets to it. Whether the thorium penetrates as a metal and is then oxidized, or whether penetration and oxidization take place concurrently, is not definitely known. However that may be, the desired result is obtained in my process, namely, the formation of thorium oxide uniformly distributed in the tungsten body. Since the thoriating is done by a coating and firing procedure, it will be apparent that any kind of prefabricated cathode structure, regardless of the size or shape of its emitter surface, may be thoriated by my process.

After being thoriated the emitter body is preferably carburized. This may be done in the conventional manner used in connection with ordinary thoriated tungsten filaments, as by heating in an atmosphere containing carbon such as a hydrocarbon atmosphere. With a surface type cathode, however, such as a cylindrical emitter body for an indirectly heated cathode, I prefer to carburize by coating the surface with powdered carbon and then heating the coated body to about 2100 C. brightness temperature in a low pressure atmosphere such as in a vacuum furnace. The carbon coating is conveniently applied by a spray gun, in which case finely divided carbon is preferably suspended in a vehicle of amyl acetate-gun cotton lacquer of a proper consistency for spraying. During the heating step the carbon combines with the tungsten to form a tungsten carbide layer adjacent the surface of the emitter body. As is the case with an ordinary carburized thoriated tungsten filament the carbide layer provides a source of carbon for reducing the thorium oxide during operation of the electron emitter. The advantage of carburizing by the carbon coating process is that a uniform carbide layer is tarmed, extending over the entire surface of the emitter After carburizing, the emitter body or cathode is ready for mounting on its supporting stem for assembly along with other suitable electrodes in an envelope. If an indirectly heated cathode structure is employed the cathode or emitter body may be heated by any suitable means as by an associated heater or by electron bombardment.

The tube with its assembled electrodes may then be exhausted and otherwise handled with ordinary tube making techniques. As hereinbefore mentioned, my improved process is adaptable for use with filamentary type struc tures as well as with surface type cathodes, either of the directly or indirectly heated varieties.

From operational and life tests using both directly heated filament structures and indirectly heated cylindrical cathodes up to several inches in diameter, 1 find the processed emitters have an electron current density and emis- Al. sion life which is as good or better than that achieved with an ordinary thoriated tungsten filament. With the cathode making process herein described, it is possible to design electron tubes having materially improved performance characteristics particularly for operation in the higher frequency ranges and at higher power levels.

I claim:

1. T he method of making a thoriated type electron emitter, which comprises coating a body of tungsten with thorium hydride, then heating the body in a low pressure atmosphere containing oxygen, and then heating the body in the presence of carbon.

2. The method of making a thoriated type cathode, which comprises forming the cathode of tungsten, coating the cathode with thorium hydride, then heating the cathode in a low pressure atmosphere containing oxygen, then recoating the cathode with carbon, and then again heating the cathode.

References Qited in the file of this patent UNITED STATES PATENTS Number Name Date 1,667,471 Friederick Apr. 24, 1928 1,670,487 Thomas May 22, 1928 1,695,819 ONiell Dec. 18, 1928 1,732,326 Cooper Oct. 22, 1929 1,850,809 Robinson Mar. 22, 1932 1,896,61 Gehrts Feb. 7, 1933 

2. THE METHOD OF MAKING A THORIATED TYPE CATHODE WHICH COMPRISES FORMING THE CATHODE OF TUNGSTEN, COATING THE CATHODE WITH THORIUM HYDRIDE THEN HEATING THE CATHODE IN A LOW PRESSURE ATMOSPHERE CONTAINING OXYGEN, THEN RECOATING THE CATHODE WITH CARBON, AND THEN AGAIN HEATING THE CATHODE. 